This invention relates to the preparation of 2-hydroxy-4-methylthiobutanoic acid and more particularly to an improved process for preparing an aqueous solution comprising 2-hydroxy-4-methylthiobutanoic acid.
2-hydroxy-4-methylthiobutanoic acid, commonly referred to as the hydroxy analog of methionine and also known as 2-hydroxy-4-methylthiobutyric acid or HMBA, is an analog of the essential amino acid methionine. Methionine analogs such as HMBA are effective in supplying methionine for nutritional uses, particularly as a poultry feed supplement. To efficiently produce feed supplements containing HMBA, the hydrolysis must be sufficiently complete.
HMBA has been manufactured by various processes involving hydrolysis of 2-hydroxy-4-methylthiobutanenitrile (also known as HMBN or 2-hydroxy-4-methylthiobutyronitrile and hereinafter "HMBN" or "nitrile"). HMBA has been produced as a racemic D,L-mixture by hydrolyzing HMBN with a mineral acid, precipitating the acid residue by addition of an alkaline earth hydroxide or carbonate, and recovering a salt of HMBA from the aqueous phase by evaporative crystallization, as described, for example, in Blake et al U.S. Pat. No. 2,745,745.
British Patent No. 915,193 describes a process for the preparation of the calcium salt of HMBA in which HMBN is hydrolyzed to HMBA in a continuous back-mixed reactor using a dilute sulfuric acid solution, and HMBA is separated from the reaction liquor by extraction with an ether. Because of the use of a continuous back-mixed reaction system, the process of the British patent may not achieve complete conversion of HMBN or amide intermediate to HMBA. The presence of unreacted material is undesirable where a liquid HMBA product is to be made.
Recently, HMBA has been commercially produced by hydrolyzing HMBN with sulfuric acid to form a high quality hydrolyzate containing HMBA, extracting HMBA from the hydrolyzate, and recovering the HMBA from the extract as described by Ruest et al. U.S. Pat. No. 4,524,077. In the process, HMBN is mixed with sulfuric acid having a strength of between about 50% and about 70% by weight on an organic-free basis at a temperature of between about 25.degree. C. and about 65.degree. C. To control the rate of reaction, the HMBN is preferably added to the acid over a period of about 30 to about 60 minutes. Under the preferred conditions, substantial conversion of the nitrile to 2-hydroxy-4-methylthiobutanamide (also known as 2-hydroxy-4-methylthiobutyramide and hereinafter "amide") takes place in a period of between about one-half hour and about one and one-half hours. Thereafter, the amide is converted to HMBA by further hydrolysis at a temperature within the range of between about 70.degree. C. and 120.degree. C. Final hydrolysis of the amide to the acid is carried out in sulfuric acid having an initial strength of between about 30% and about 50% by weight on a organic-free basis. To provide the preferred acid strength, the acid phase is diluted by adding water before heating the reaction mixture. Under conditions of relatively dilute acid strength and increased temperature, the amide is converted to the acid within a period of approximately one and one-half to three hours. In carrying out the hydrolysis, approximately one mole of sulfuric acid per mole of the HMBN feed is used, with an acid excess of 0 to 10%, preferably 0 to 5%, providing satisfactory results. Ruest et al. describe a batch process and state that a batch process is preferred to ensure that the hydrolysis reaction is carried substantially to completion. If a continuous reaction system is utilized, Ruest et al. describe that it should be designed and operated to assure essentially complete conversion. For example, continuous operation could be implemented in a plug flow tubular reactor or cascaded stirred tank system. A single back-mixed reactor is described by Ruest et al. as providing adequate conversion only at residence times that would generally be considered unacceptable for commercial production.
Hernandez et al. U.S. Pat. No. 4,912,257 describes a process in which HMBA is produced by sulfuric acid hydrolysis of HMBN in a single step. HMBN is fed to an acidification vessel where it is mixed with 98% sulfuric acid at an acid/nitrile molar ratio between 0.5 and 2 to form a reaction mixture containing 20-50% by weight sulfuric acid. The mixture is agitated and cooled to 50 .degree. C. in a continuous addition loop for 30-60 minutes as the reaction mixture is produced batchwise. The reaction mixture is then fed to a hydrolysis reactor and heated to a temperature of between 60.degree. C. and 140.degree. C. for five minutes to six hours while applying a slight vacuum to the reactor. The process described by Hernandez et al. is said to produce HMBA by hydrolysis of the acidified HMBN solution in a single step unlike the two step hydrolysis processes known in the art.
In order to provide a high quality hydrolyzate solution containing maximum HMBA and minimal nitrile and amide components, high conversion of HMBN and 2-hydroxy-4-methylthio-butyramide to HMBA must be obtained. Batch production of HMBA generally provides high conversion. However, conventional batch processes for producing HMBA have several drawbacks. The productivity of a batch process is limited by batch turnaround time. Additionally, the quality of HMBA hydrolyzate can deviate between batches because reaction conditions can vary as each batch is produced. Filling and emptying of the batch reactor and non-steady state conditions cause vapor emissions which must be treated before release. The equipment required for the prior art processes is costly. Sulfuric acid and water are mixed in an acid dilution tank to form diluted sulfuric acid feed. A heat exchanger is required to remove the heat of dilution that is generated within the tank. The tank, heat exchanger, pump and recirculation loop must be of corrosion resistant construction.